The present invention relates generally to suspension systems for motor vehicles and, more particularly, to a suspension system in which damping is controlled either passively, semi-actively or actively and which defines a fail-safe damping rate for the suspension in the event of failure within the suspension system.
When a motor vehicle is driven, springs in its suspension system compress and expand to absorb shocks which would otherwise be transmitted to occupants of the vehicle. Once deflected, the springs continue to oscillate until they eventually return to their original state. Since spring oscillations create handling problems and reduce ride comfort of the vehicle, shock absorbers are used to dampen the oscillations of the springs and thereby stabilize the suspension system and return the springs to their original state substantially more quickly.
Dependent on the road conditions and driving preferences of an operator of a vehicle, the damping rate defined by shock absorbers may vary from slow or soft, for a smoother ride, to fast or hard, for better handling and control of the vehicle. Normally, an intermediate damping position is selected as a compromise between ride comfort and vehicle handling. Since driving conditions and operator preferences vary, a number of suspension systems have been developed which allow the operator of a motor vehicle including the suspension system to select the spring damping characteristics of the shock absorbers. Variable suspension systems often permit an operator to select a "soft" suspension or a "hard" suspension depending on the current road conditions and driver preference. Most variable suspension systems also provide a "normal" damping selection which is somewhere intermediate the soft and hard selections and closely akin to the conventional compromise damping provided by systems which cannot be varied.
In variable damping suspension systems, there is a concern for what damping rates will be applied in the event of a failure within the system. For example, if one shock absorber is set to hard and the remaining three shock absorbers are set to soft, the handling characteristics of the vehicle will be substantially less than desirable. A number of solutions have been proposed to overcome these failure problems in motor vehicle variable damping suspension systems.
U.S. Pat. No. 4,526,401 proposes one such solution for a three position variable damping suspension system wherein the time between a commanded damping level and the attainment of that level is monitored with a fault condition being identified for excessive time periods. If a fault is identified, a damping force command signal for all shock absorbers of the system is set to a normal or medium damping force such that the difference in damping force between a failed shock absorber and the remaining shock absorbers is no more than one damping level.
U.S. Pat. No. 4,666,180 discloses a variable damping force motor vehicle suspension system wherein the operating time of actuators of all shock absorbers of a vehicle are monitored by a first circuit. If the operating time of the first circuit is exceeded, power is still provided from a second circuit in a continuing attempt to set a failed shock absorber. If a time period defined by the second circuit is exceeded, the power is removed from the system. In the best case of either of the disclosed failure handling arrangements, it is probable that the suspension system will be disabled in a state with one shock absorber applying a damping rate which is different than the other three shock absorbers, possibly substantially different.
U.S. Pat. No. 4,815,575 discloses a motor vehicle shock absorber with a controlled variable damping rate wherein the damping of the vehicle suspension system is controlled by providing a three phase alternator which is connected to a resistor through a rectifier and chopper circuit. By monitoring the motion of the suspension, a secondary control current is provided to the resistor via the chopper circuit to control the power which is generated in the alternator of the suspension system and dissipated in the resistor to thereby control the damping rate of the suspension. Unfortunately, in the event of failure within the control circuit which disables the chopper circuit, the shock absorber operates free of any electrically induced damping such that it is reduced to an ineffectively low damping rate defined by the mechanical and frictional forces inhibiting movement of the shock absorber.
It is thus apparent that a need exists for a controlled damping motor vehicle suspension system which defines a fail-safe damping rate for the suspension in the event of failure of the suspension. In such a suspension system, the damping rate of all of the shock absorbers would be set to the fail-safe damping rate under substantially all failure modes. Preferably, such a system would be electrically operable to substantially preclude inhibiting the fail-safe feature by mechanical failure with a shock absorber.